High above the thin Martian skies, NASA’s MAVEN spacecraft is carrying out a mission: determine how Mars lost its early atmosphere, and with it, its water. While previous Mars orbiters have peered down at the planet’s surface, MAVEN is spending part of its time gazing at the stars, looking for subtle changes in their color as they dip through the limb of Mars and set below the horizon. Such stellar occultations reveal what the atmosphere is made of, and how its composition varies with altitude. MAVEN’s observations are providing the most detailed picture of the Mars upper atmosphere to date, helping scientists understand how Mars turned from a warm and wet planet in its youth, into the forbidding desert that we see today.